Study of the impact of structural factors and cleaning efficiency in reducing hazardous litter density and its related pollution in urban environment

Cigarette filter is the most common hazardous litter that contains many pollutants including PAHs. The durability of this litter in the urban environment has an important effect on the rate of pollutant leakage. In this study, the leakage rate of PAHs from the littered cigarette filters was estimated by considering the affecting parameters on their durability in the urban environment. The results showed that the density of littered cigarette filters in the studied locations was 0.00048–0.13563 g/m2. The maximum spatial variation of the littered cigarette filter was 225 times. The average leakage of the total studied PAHs was estimated to be 2.048 µg/10 m2. The impact of structural factors and efficiency of urban cleaning in the estimated leakage was at most 2.4 times. It is necessary to change the behavior of citizens in littering the cigarette filter, considering its durability in the urban environment, to reduce the environmental and health consequences caused by the leakage of PAHs.

One of the consequences of increasing urbanization and globalization in recent decades is the growth of urban solid waste generation 1,2 .The variety of available products and services due to industrial development and the increase in prosperity in the communities has also caused the composition of municipal solid waste to become more diverse 3 .In this situation, the emergence of new solid wastes with special characteristics is one of the challenges of municipal solid waste management 3 .Some types of emerging solid wastes in recent decades, including cigarette filters, are classified as hazardous wastes due to characteristics such as the leakage of chemical and toxic compounds 4,5 .
Seven decades ago, the filter was added to previous cigarettes with the aim of reducing the entry of cigarette smoke pollutants into the lungs of smokers, and today the most form of tobacco consumption is filtered cigars 6,7 .Although the filter has the ability to trap cigarette smoke pollutants and can be effective in reducing the harm to smokers, the generation of an emerging solid waste has been one of the consequences of filtered cigars 8 .Littering the cigarette filter as a solid waste caused by smoking is a common behavior by smokers 9,10 .This method of waste disposal has caused the cigarette filter to be reported as a widespread litter in public environments such as beaches and urban environments 11 .A study of different cities and beaches showed that cigarette filters have a significant contribution to the composition of litter.For example, studies carried out in Argentina, Germany, Spain, Brazil, and Iran reported that cigarette butts were the most frequent litter in the studied environments [12][13][14][15] .The small size and low visibility of cigarette filters in public and urban environments, especially on beaches, makes it very difficult to collect them 16 .In this situation, the durability of this litter in the environment will increase.
Increasing the durability of the cigarette filter in the environment will increase the leakage of trapped pollutants.Due to the presence of numerous chemicals in cigarette smoke, it is expected that the pollutants trapped in the cigarette filter very diverse 15 .For example, nicotine is one of the known toxin in the cigarette filter, which quickly leaks from it 14 .Also, heavy metals including chromium, cadmium, nickel, mercury, and lead have been detected in various amounts in cigarette filters, which have been proven to leak from it 15 .PAHs are important pollutants in cigarette filters that leak into the environment due to climatic conditions such as humidity 17 .In past studies, all types of PAHs have been detected in cigarette filters 17,18 .However, the leakage of PAHs from the cigarette filter to the environment depends on several factors, including the density of the littered cigarette filter, climatic conditions, durability of the littered cigarette filter in the environment, and the initial concentration of the pollutant.The behavior of smokers in disposing of cigarette filters, the structural characteristics of the environment, and the efficiency of the urban cleaning system have a direct impact on the mentioned parameters.The urban cleaning system in the studied city is similar to the pattern used throughout Iran.In this pattern, people called "pakban" collect litter manually using long brooms.Due to the possibility of resuspension of deposited particles on the surface and also to increase the speed of the process, this activity is carried out in the late hours of the night when there is minimal presence of citizens in the urban environment.Therefore, the aim of this study was to estimate the leakage of PAHs from the littered cigarette filter in the urban environment by considering the density of the littered cigarette filter, the structural characteristics of the environment, and the efficiency of the urban cleaning system.

Method
Data of the density of littered cigarette filters were collected by the method of visual survey of urban environments 16 .In this method, the defined surface including the width of the sidewalk plus one meter of the street width was visited and the number of littered cigarette filters was counted 16,19 .The evaluation of the density of littered cigarette filters in the studied locations was done in the evening hours and on working days 15,20 .To detect the average weight of littered cigarette filters, freshly smoked cigarette filters from ten popular cigarette brands in Iran were weighed 20 .As shown in Fig. 1, after collecting data of the density of littered cigarette filters, Cigarette Butt Pollution Index (CBPI) was used to interpret the status of the studied locations 20,21 .The details of the calculation of this index are shown in Table 1.Based on this, to calculate CBPI, applying E coefficient on the density of the littered cigarette filters was used.
The leakage of PAHs from the cigarette filter was estimated according to the initial concentration of these pollutants in the cigarette filter and its leakage potential in different climatic conditions 22 .The initial concentration of sixteen types of PAHs in cigarette filter has been reported by 0.48-5.76µg/g 18 .Also, the rate of leakage of various types of PAHs from cigarette filters has been reported by 1-49%, which will increase under the influence of humidity 18 .Therefore, by identifying the density of littered cigarette filters in each location and the reported concentrations, the amount of PAHs leakage was estimated in each of the studied locations.

Results and discussion
The results showed that there was no separate program for collecting littered cigarette butts in the studied city.Separate trash bins for cigarette butts were not installed in the urban environment.Littered cigarette butts were collected by the employees of the urban cleaning system (Pakbans) in the mass of littered wastes.In this system, the cleaning of low access points, such as tree pits and runoff collection channels, was often missed.The density of littered cigarette filters in the ten studied locations is shown in Table 2.The results showed that the density of cigarette filters was not the same in the studied locations and temporal and spatial variation were observed.The results showed that the minimum and maximum density of the littered cigarette filter were 0.00048 and 0.13563 g/ m 2 , respectively.Spatial variation in the density of littered cigarette filters were evident.The average density in recreational land-use, residential land-use, and commercial land-use was 0.00051, 0.02349, and 0.11411 g/m 2 , respectively.The difference observed in the density of littered cigarette filters is directly related to the land-use.The effect of land-use on population density causes the possibility of cigarette filter littering in commercial land-use more than in residential and recreational land-uses 23 .However, in some places, the density of littered cigarette filters may be higher than the local average 24 .For example, in residential land-use, the number of littered cigarette filters is higher in places such as around supermarkets 16 .Also, more littered cigarette filters have been reported in recreational land-uses such as parks around benches 25 .These are points with high potential for cigarette filter littering and have an important effect on average density.Also, the phenomena affecting the population in the urban environment cause a change in the number and composition of litter, including cigarette filters.For example, one consequence of the Covid-19 pandemic was the reduction of people in the urban environment and the reduction of the possibility of waste littering 26,27 .In this situation, the difference in the number of littered cigarette filters in different land-uses will change.
The density of littered cigarette filters has a direct effect on the leakage of trapped pollutants into the environment 22 .However, the effect of climatic conditions and the durability of cigarette filters are also effective in the leakage of trapped pollutants 9 .This effect is shown by comparing the density of cigarette filters with CBPI.As shown in Fig. 2, the density changes in the studied locations were different from the CBPI changes.The results showed that the lowest and highest CBPI were 0.1 and 10.850, respectively.Based on this, 30% of the studied locations were in significant pollution and worse status.The average CBPI in commercial land-use, residential land-use, and recreational land-use was 8.05, 1.21, and 0.106, respectively.The reason for the difference in CBPI is the difference in environmental conditions, which will cause a change in the E coefficient 28 .For example, the presence of three pits and also the surface runoff collection channels will increase the E coefficient 20 .The reason for this effect is the decrease of the efficiency of the urban cleaning system in these points, which are known as "low access points" 29 .Therefore, the increase of these points, which leads to an increase in the durability of littered cigarette filters due to the reduction of the efficiency of the cleaning system, is effective in increasing the leakage of trapped pollutants into the environment 20 .In addition, climatic conditions such as increasing humidity are effective in changing the E coefficient and increasing CBPI.Although the climatic conditions were the same in the studied area, however, the increase in humidity in recreational land-uses such as parks due to daily irrigation caused E coefficient increase compared to residential and commercial land-uses 20 .
Therefore, CBPI is a better index compared to the density of littered cigarette filters due to considering the factors affecting pollutant leakage into the environment 28 .The results of the leakage of various PAHs into the environment in the studied locations are shown in Table 3.The average leakage of these pollutants in the studied area was estimated at 0.204 µg/m 2 .The quantity of leakage of different types of PAHs into the environment was not the same, so that the highest and lowest amount of leakage were related to Naphthalene (0.1557 µg/m 2 ) and Chrysene (0.00049 µg/m 2 ), respectively.Also, due to the two effective factors in pollutant leakage into the environment, including density and structural-environmental factors, the variation in the amount of leakage in the studied land-uses was expected 15 .The results showed that the average leakage of total PAHs in commercial land-use, residential land-use, and recreational land-use was 0.426 µg/m 2 , 0.085 µg/m 2 , and 0.016 mg/m 2 , respectively.Based on this, littered cigarette filters are a source of PAHs leakage into the urban environment, which transfer different amounts of pollutants according to the initial concentration in the filter, the density of the filters, and the environmental conditions.Table 1.Formula, variables, and status classification of CBPI as used index in this study 20  Although the importance of this hazardous waste may be ignored due to its small size and the small amount of leakage per littered cigarette filter, however, considering the annual generation of eight trillion cigarette filters, its importance will be more understood 30 .Therefore, the abundance and dispersion as two characteristics of the cigarette filter make it a special source of leakage of various pollutants into the environment, including PAHs.A serious concern about pollutant leakage from littered cigarette filters is the contamination of soil and water resources.In previous studies, it has been proved that various types of pollutants, including nicotine, can leak from cigarette filters into water resources 14 .Also, in case of correct disposal of this waste by smokers and finally its landfill with municipal solid waste, there is a possibility of increasing landfill leachate pollutants, which has been analyzed and reported in the case of heavy metals 31 .Therefore, the health and environmental consequences caused by the leakage of trapped pollutants in cigarette filters are one of the reasons for paying attention to the management of this hazardous waste.For example, the consequences of exposure to PAHs include weakened immune systems, birth defects, genetic damage, damage to the nervous system, disruption of reproductive and hormonal systems, and cancer 32 .
The results showed that in addition to creating unpleasant landscapes in the urban environment, littered cigarette filters were an environmental threat due to the leakage of PAHs.Therefore, it is necessary to reduce the littering of cigarette filters by smokers due to the presence of low access points in urban environments and the low efficiency of the urban cleaning system in collecting littered cigarette filters 33 .Increasing the awareness of citizens and smokers about the environmental and health consequences related to littered cigarette filters can be effective in correcting their attitudes and changing behavior leading to a reduction in littering cigarette filters 10 .This approach is an effective act in "reduction step" of littered cigarette filters in the environment 16 .In addition, the reduction of littered cigarette filters will be provided by improving waste management structures, including the installation of special trash bins for the disposal of cigarette filters 30 .Considering the high proportion of cigarette filter littering even in cases where trash bins are installed in sufficient quantity and with short distance in the urban environment 14 , increasing the efficiency of the urban cleaning system as the most important approach in the "removal step" is necessary.For this purpose, it can be useful to use cleaning equipment adapted to urban structural characteristics and increase access, especially to low access points.

Conclusion
The effect of the presence of low access points and the efficiency of the urban cleaning system on the density of littered cigarette filters and the leakage of PAHs into the urban environment was studied.The results showed that the average density of littered cigarette filters in commercial, residential, and recreational land-uses was 0.11411, 0.02349, and 0.00051 mg/m 2 , respectively.The cigarette butt pollution index in the studied locations was 0.1-10.850,which showed that 30% of the studied locations were in a very low pollution status and 50% of the studied locations were in a significant pollution and worse status.The average leakage of PAHs from the cigarette filter to the urban environment was estimated to be 0.2048 µg/m 2 .Accordingly, littered cigarette filters are a numerous and disperse source of PAHs in urban environments.In order to prevent health and environmental consequences caused by PAHs and other pollutants leaked from littered cigarette filters, it is necessary to increase the awareness of smokers and improve their behavior to reducing the proportion of cigarette filter littering in the urban environment.Also, the use of urban cleaning equipment with higher efficiency, especially in increasing the efficiency of cigarette filter collection in low access points, can reduce the durability of cigarette filters in the environment and reduce the uncontrolled leakage of pollutants.

Figure 1 .
Figure 1.The order of the four stages of this study.